(1) Field of the Invention
The present invention relates to a process for the conversion of ethylbenzene in a C.sub.8 aromatic hydrocarbon mixture. More particularly, the present invention relates to a process for the hydrode-ethylation of ethylbenzene to benzene.
(2) Description of the Related Art
From the industrial viewpoint, p-xylene is now the most important of xylene isomers. p-Xylene is generally prepared from a C.sub.8 aromatic hydrocarbon mixture obtained by subjecting naphtha to reforming and aromatic extraction and fractional distillation or from a C.sub.8 aromatic hydrocarbon mixture obtained by subjecting cracked gasoline obtained as a by-product from a thermal cracking of naphtha to aromatic extraction and fractional distillation. Although the composition of this starting C.sub.8 aromatic hydrocarbon mixture varies over a wide range, the mixture generally comprises 10 to 40% by weight of ethylbenzene, 12 to 25% by weight of p-xylene, 30 to 50% by weight of m-xylene and 12 to 25% by weight of o-xylene. The physical properties of the respective components of the C.sub.8 aromatic hydrocarbon mixture are as follows.
______________________________________ Melting Point (.degree.C.) Boiling Point (.degree.C.) ______________________________________ Ethylbenzene -94.4 136.2 p-Xylene 13.4 138.4 m-Xylene -47.4 139 o-Xylene -28.0 142 ______________________________________
In the industrial process for the preparation of p-xylene, in general, the starting C.sub.8 aromatic hydrocarbon mixture is first fed to the p-xylene-separating step, where p-xylene is separated and recovered. As pointed out hereinbefore, the boiling point of p-xylene is very close to the boiling point of m-xylene, and therefore, separation by distillation is industrially disadvantageous. Accordingly, separation is accomplished by a deep freeze separation process utilizing the difference of the melting point or the adsorptive separation process in which p-xylene is selectively adsorbed in a porous solid adsorbent. The remaining C.sub.8 aromatic hydrocarbon mixture in which the p-xylene content has been reduced at the p-xylene-separating step is fed to the isomerizing step, where isomerization is carried out so that a p-xylene concentration corresponding substantially to the thermodynamic equilibrium composition is attained. Then, the isomerized mixture is recycled to the p-xylene-separating step together with a fresh starting C.sub.8 aromatic hydrocarbon mixture. The circulation system comprising the above-mentioned p-xylene-separating step and xylene-isomerizing step is called "separation-isomerization cycle" hereinafter. Note, if circumstances require same, o-xylene is separated and recovered by distillation.
As pointed out hereinbefore, a considerable amount of ethylbenzene is contained in the C.sub.8 aromatic hydrocarbon mixture. Accordingly, to prevent an accumulation of ethylbenzene in the separation-isomerization cycle, ethylbenzene is removed and ethylbenzene in an amount determined by the removal ratio is circulated in the separation-isomerization cycle.
As the amount of ethylbenzene circulated in the separation-isomerization cycle is small, the amount of circulated liquid is reduced and the energy consumption required at each of the p-xylene-separating step and xylene-isomerizing step is reduced, and thus a great economical advantage is obtained. Nevertheless, according to the conventional technique, it is difficult to reduce the amount of ethylbenzene circulated in the separation-isomerization cycle by an inexpensive method, to an extent such that the ethylbenzene content can be regarded as substantially zero.
According to the method customarily adopted for removing ethylbenzene, an isomerizing catalyst having an ethylbenzene-converting activity is used at the isomerizing step whereby ethylbenzene is converted to xylene or a substance that can be easily separated from xylene at the isomerization reaction. For example, there can be mentioned (1) a method in which ethylbenzene is converted to xylene by a dual-functional catalyst comprising a platinum and a solid acid (U.S. Pat. No. 3,409,699), (2) a method in which ethylbenzene is converted to benzene and diethylbenzene by transalkylation reaction (U.S. Pat. Nos. 3,856,871 and 4,120,908), and (3) a method in which ethylbenzene is converted to benzene by de-ethylation reaction (European Patent No. 138,617). According to methods (1) and (2), in view of the reaction principle, it is difficult to increase the conversion of ethylbenzene. According to method (3), in view of the reaction principle, it is possible to increase the conversion of ethylbenzene, but even if the conversion of ethylbenzene can be practically increased, if the conventional flow for converting and removing ethylbenzene in the separation-isomerization cycle is used, since ethylbenzene is contained in a relatively large amount in the starting C.sub.8 aromatic hydrocarbon mixture, reduction of the ethylbenzene concentration at the p-xylene-separating step is limited, and it is difficult to drastically reduce the ethylbenzene concentration in the liquid circulated in the separation-isomerization cycle.
As one feasible selection, U.S. Pat. No. 4,159,282 discloses a method in which the majority of ethylbenzene contained in the starting C.sub.8 aromatic hydrocarbon mixture is converted by using a zeolite as a catalyst in an independent reaction vessel before the starting C.sub.8 aromatic hydrocarbon mixture is fed to the separation-isomerization cycle, but no specific example is shown therein. The invention disclosed in this U.S. patent is characterized in that a crystalline aluminosilicate zeolite having a crystal size of at least 1 micron and a silica/alumina ratio of at least 12 not only isomerizes xylene but also selectively converts ethylbenzene. However, in all of the specific examples, the highest conversion of ethylbenzene is only 43.5%.
Namely, according to the conventional techniques, in view of the reaction principle, it is impossible to increase the conversion of ethylbenzene, or even where there is no upper limit to the conversion of ethylbenzene in view of the reaction principle, an increase of the conversion of ethylbenzene tends to result in an increase of the loss of xylene, and as the conversion of ethylbenzene approaches 100%, the loss of xylene is drastically increased, with the result that the increase of the conversion of ethylbenzene with a small loss of xylene cannot be attained.